When scientists talk about the cryosphere, they mean the places on Earth where water is in its solid form, frozen into ice or snow. Read more ...

On Wednesday, November 21 from 9:00 a.m. to 10:00 a.m. (USA Mountain Time), the following data collections will not be available due to planned system maintenance: AMSR-E, Aquarius, ASO, High Mountain Asia, IceBridge, ICESat/GLAS, MEaSUREs, MODIS, NISE, SMAP, SnowEx, and VIIRS.

This data set, part of the NASA Making Earth System Data Records for Use in Research Environments (MEaSUREs) program, is an improved, enhanced-resolution, gridded passive microwave Earth System Data Record (ESDR) for monitoring cryospheric and hydrologic time series from SMMR, SSM/I-SSMIS, and AMSR-E. The Calibrated Passive Microwave Daily EASE-Grid 2.0 (CETB) gridded data use the most mature available Level-2 satellite passive microwave records from 1978 to mid-2017.

The data are a new, multi-sensor Level 3 Earth Science Data Record (ESDR) with recently released improvements in cross-sensor calibration and quality checking, modern file formats, better quality control, improved projection grids, and local time-of-day (LTOD) processing. These data are gridded to the EASE-Grid 2.0 definition and include enhanced-resolution imagery, as well as coarse-resolution, averaged imagery.

Format

The data are in NetCDF (.nc) format, using CF 1.6 (Climate and Forecast) and ACDD 1.3 (Attribute Conventions for Dataset Discovery) metadata conventions.

Background color on

File Naming Convention

Following is the file naming convention for this data set (Table 1) with an example file name.

Individual CETB file sizes range from 0.08 MB to 102.6 MB. File sizes vary because CETB files employ internal compression. A full day of data ranges in size from 13.06 MB to 9.79 GB, with an average of 3.94 GB per day.

Spatial Resolution

Each channel is processed at conventional (25 km) and enhanced resolutions depending on frequency. The coarsest grid resolution is 25 km, with enhanced-resolution grids defined in a nested fashion in powers of 2, at 12.5, 6.25 and 3.125 km (see Figure 1.) (Brodzik and Long, 2016). The expected level of resolution enhancement for the CETB products is channel-dependent, at best 3.125 km (Long and Brodzik, 2016).

Projection and Grid Description

The data are gridded to EASE-Grid 2.0 projections, at various coverages and spatial resolutions as defined in Table 2. All channels are gridded to 25 km and the higher resolutions depend on channel frequency as follows: frequencies below 12 GHz are at 12.5 km, between 12 and 30 GHz, the resolution is 6.25 km, and above 30 GHz, the enhanced-resolution is at 3.125 km.

Name

Projection

Resolution (km)

Columns

Rows

Latitude Extent (degrees)

Longitude Extent (degrees)

Table 2. CETB EASE-Grid 2.0 Projections and Grid Dimensions

EASE2_N25km

Northern Lambert Azimuthal

25

720

720

0 - 90

-180 - 180

EASE2_N12.5km

Northern Lambert Azimuthal

12.5

1440

1440

0 - 90

-180 - 180

EASE2_N6.25km

Northern Lambert Azimuthal

6.25

2880

2880

0 - 90

-180 - 180

EASE2_N3.125km

Northern Lambert Azimuthal

3.125

5760

5760

0 - 90

-180 - 180

EASE2_S25km

Southern Lambert Azimuthal

25

720

720

-90 - 0

-180 - 180

EASE2_S12.5km

Southern Lambert Azimuthal

12.5

1440

1440

-90 - 0

-180 - 180

EASE2_S6.25km

Southern Lambert Azimuthal

6.25

2880

2880

-90 - 0

-180 - 180

EASE2_S3.125km

Southern Lambert Azimuthal

3.125

5760

5760

-90 - 0

-180 - 180

EASE2_T25km

Cylindrical Equal-Area

25.02526

1388

540

+/-67.0575406

-180 - 180

EASE2_T12.5km

Cylindrical Equal-Area

12.51263

2776

1080

+/-67.0575406

-180 - 180

EASE2_T6.25km

Cylindrical Equal-Area

6.256315

5552

2160

+/-67.0575406

-180 - 180

EASE2_T3.125km

Cylindrical Equal-Area

3.128.15750

11104

4320

+/-67.0575406

-180 - 180

Background color on

Temporal Coverage

Temporal coverage varies by sensor. See Table 3 for the actual coverages.

Sensor

Platform

Begin Coverage

End Coverage *

Table 3. Temporal Coverage by Sensor

AMSR-E

AQUA

01 June 2002

04 October 2011

SSM/I

F08
F10
F11
F13
F14
F15

07 September 1987
08 December 1990
03 December 1991
03 May 1995
07 May 1997
23 February 2000

31 December 1991
14 November 1997
16 May 2000
19 November 2009
23 August 2008
30 June 2017

SSMIS

F16
F17
F18
F19

01 November 2005
01 March 2008
08 March 2010
27 November 2014

30 June 2017
30 June 2017
30 June 2017
09 February 2016

SMMR

Nimbus

25 October 1978

20 August 1987

*End coverage represents the end date of the majority of the granules.

Temporal Resolution

The grids are produced twice daily. T grids are separated by ascending/descending passes, and N and S grids are separated by LTOD.

Table 4 shows the beginning and ending times for the LTOD morning/evening split as hours after UTC midnight on the day of processing. Note also that for all platforms except AMSR-E, the LTOD split times are the same for the Northern and Southern hemispheres. All of the N and S grids in the data set were processed with these times. These values are stored as part of the metadata in the CETB files as an attribute of the TB data.

Table 4. LTOD Times

Platform

Year

Morning start time

Morning end time

Evening start time

Evening end time

F08

All years

0.0

12.0

12.0

24.0

F10

1990-1993

2.0

14.0

14.0

26.0

F10

1994

3.0

15.0

15.0

27.0

F10

1995-1997

4.0

16.0

16.0

28.0

F11

All years

0.0

12.0

12.0

24.0

F13

All years

0.0

12.0

12.0

24.0

F14

1997-2001

3.0

15.0

15.0

27.0

F14

2002-2004

2.0

14.0

14.0

26.0

F14

2005-2008

0.0

12.0

12.0

24.0

F15

2000-2005

3.0

15.0

15.0

27.0

F15

2006-2007

2.0

14.0

14.0

26.0

F15

2008-2011

0.0

12.0

12.0

24.0

F15

2012

-2.0

10.0

10.0

22.0

F15

2013-2016

-3.0

09.0

9.0

21.0

F16

2005-2007

3.0

15.0

15.0

27.0

F16

2008-2009

2.0

14.0

14.0

26.0

F16

2010-2011

1.0

13.0

13.0

25.0

F16

2012-2013

0.0

12.0

12.0

24.0

F16

2014

-1.0

11.0

11.0

23.0

F16

2015-2017

-2.0

10.0

10.0

22.0

F17

All years

0.0

12.0

12.0

24.0

F18

All years

0.0

12.0

12.0

24.0

F19

All years

0.0

12.0

12.0

24.0

SMMR

All years

6.0

18.0

18.0

30.0

AMSR-E

All years NH

5.0

17.0

17.0

29.0

AMSR-E

All years SH

8.0

20.0

20.0

32.0

Background color on

Parameter or Variable

The parameters for this data set are listed in Table 5.

Parameter

Description

Fill Value

Missing Value

Table 5. Parameters

TB

Brightness temperature

0.0

600.00

TB_time

Average time of the measurements used to derive TB

-32768

Not used

TB_std_dev

Standard deviation of the measurements used to derive TB

655.35

655.34 (only used when TB is set to missing value)

TB_num_samples

Number of measurements used to derive TB

0

Not used

Incidence_angle

Average incidence angle of the measurements used to derive TB

-0.01 (°)

Not used

Parameter Description

Brightness temperature depends on the emissivity and physical temperature of the observed target and varies with the frequency and polarization of the passive microwave sensors. The relationship between the measured brightness temperature and the effective physical temperature of the observed target is described by the Rayleigh-Jeans approximation of Planck's equation.

Sample Data Record

Figures 2-4 are examples of AMSR-E data from 27 September 2011. There is one image per grid (Southern, Temperate & Tropical, and Northern): two images (Southern and Northern) are at a spatial resolution of 3.125 km, and the Temperate & Tropical image is at 6.25 km.

Software and Tools

Data Access Tools

For a list of resources for accessing NetCDF files, see NetCDF Software Tools. Geolocation files for this data set are NetCDF (.nc) files and are located here: ftp://sidads.colorado.edu/pub/tools/easegrid2/. File names indicate the grids for each file: Northern (N), Southern (S), or Temperate & Tropical (T). As an example, EASE2_N12.5km.geolocation.v0.9.nc is for the 12.5 km resolution in the Northern Hemisphere.

Other tools for working with the data will be available shortly.

Background color on

Sensor or Instrument Description

For a detailed description of the instruments used to acquire the data, see the following NSIDC web sites:

Table 6 provides a list of the channels for each instrument. All channels are gridded to 25 km and the higher resolutions depend on channel frequency. Frequencies below 12 GHz are at 12.5 km, between 12 and 30 GHz, resolution is 6.25 km, and above 30 GHz, resolution is 3.125 km.

There are two general processing steps in generating the CETB product. These include data set pre-processing for spatial and temporal selection, and gridding/reconstruction of the data. See the appropriate sections below.

Data set preprocessing
The first stage of processing ingests the raw swath TB and performs initial data and temporal selections. Only the highest quality TB measurements are used to ensure the most reliable data set. Swath data are mapped to output grids by measurement geolocation and LTOD.

Local Time-of-Day: All of the CETB passive microwave sensors fly on near-polar, sunsynchronous satellites, which maintain an orbital plane with an orientation that is (approximately) fixed with respect to the sun. Thus, the satellite crosses the equator on its ascending (northbound) path at approximately the same LTOD. The resulting coverage pattern yields passes about 12 hours apart in LTOD at the equator. Most areas near the pole are covered multiple times per day. Analysis shows that the data from a single sensor fall into two LTOD ranges for polar measurements. The two periods are typically less than 4 hours long, spaced 8 or 12 hours apart. Significantly, due to the orbit repeat cycle, two succeeding days at any particular location may make measurements at different LTOD, and therefore, at different times during the diurnal cycle (Gunn, 2007), introducing undesired variability (noise) into a time series analysis.

The CETB azimuthal (Northern or Southern) grids are split into two images per day based on the LTOD approach of Gunn and Long (2008). This ensures that all measurements in any one image have consistent spatial/temporal relationships. The CETB adopts the LTOD division scheme for the Northern and Southern hemispheres. In the equatorial regions of the EASE2-T grids, LTOD is equivalent to division by ascending vs. descending passes,

Each file includes gridded arrays of the following variables: brightness temperature, number of contributing measurements, as well as the average time, standard deviation, and average incidence angle of contributing measurements used to derive the TB at each pixel. This enables investigators to explicitly account for the LTOD temporal variation of the measurements included in a particular pixel.

Gridding/Reconstruction

CETB products are generated on coarse resolution grids for all channels using a low-noise “drop-in-the-bucket” average, and enhanced-resolution grids using rSIR (radiometer version of Scatterometer Image Reconstruction) image reconstruction techniques (Long and Brodzik, 2016). For enhanced-resolution grids, the effective resolution depends on the number of measurements and the precise details of their overlap, orientation, and spatial locations. See the Derivation Techniques and Algorithm section for more information.

Antenna Pattern and Measurement Spatial Response
For image reconstruction processing, information about the antenna gain pattern, the scan geometry, and integration period are required to compute the effective measurement response function (MRF). The MRF describes how much the emissions from a particular receive direction affect the observed TB value. For each sensor and channel, the MRF is modeled as a two-dimensional Gaussian using the 3-dB footprint size (Long and Brodzik, 2016). See Table 8 for the field-of-view values.

Sensor

Frequency (GHz)

Semi-major (km)

Semi-minor (km)

Table 8. Effective Field of View

SSMI

19 H, V

69

43

SSMI

22 V

60

40

SSMI

37 V

37

28

SSMI

37 H

37

29

SSMI

85 H, V

15

13

SSMIS

19 H, V

72

44

SSMIS

22 V

72

44

SSMIS

37 H, V

44

26

SSMIS

91 H, V

15

9

AMSR-E

6 H, V

75

43

AMSR-E

10.7 H, V

51

29

AMSR-E

18 H, V

27

16

AMSR-E

23 H, V

32

18

AMSR-E

36 H, V

14

8

AMSR-E

89 (H or V)

7

4

AMSR-E

89 (H or V)

6

4

SMMR

6.6 H, V

121

79

SMMR

10.7 H, V

74

49

SMMR

18 H, V

44

29

SMMR

21 H, V

38

24

SMMR

37 H, V

21

14

Known Data Problems

Empty pixels in GRD images
When no swath measurement center locations were mapped to the area of the a gridded pixel, GRD images will occasssionally have single pixels with no data. Normally, rSIR images do not suffer from this problem, because the rSIR gain threshold is set to a value that almost always ensures at least one component measurement that can be used to derive the pixel brightness temperature. However, beginning 4 Nov 2004, the AMSR-E 89 GHz A-horn developed a permanent problem that resulted in a loss of observations for the remining life of AMSR-E. After this date, the rSIR 3.125 km 89 GHz data does occasionally have missing pixels (Beitsch, Kaleschke, and Kern, 2014).

Files downloaded outside of EarthData Search may be on different days
For data acquired from https://n5eil01u.ecs.nsidc.org/MEASURES/NSIDC-0630.001/, all T grids produce data from 00:00 to 23:59.99 for date of file. N and S grids produce data that may include up to 6 hours of the day before and possibly the day after. These files are placed in the day directory that matches the beginning of the data in the file.

Example: File for day X starts a couple hours prior to UTC day X midnight. This file will be in the directory for day X-1, and there will be no file in directory for day X.

This problem does not affect users of EarthData Search.

Missing Dates
There are files for every date of each sensor's temporal coverage. For dates with no data, data files exist with all variables set to "_FillValue" (which compresses down to small files).

Background color on

Derivation Techniques and Algorithms

The following sections describe CETB gridding algorithms. Please refer to Long and Brodzik (2016) for the theory of reconstruction techniques and complete details of rSIR. Figure 5 provides a graphical representation of the enhanced resolution of rSIR measurements.

Figure 5. rSIR 3.125 km Resolution

Coarse Resolution (GRD) Gridding Algorithms
The CETB coarse resolution gridding procedure is a simple, “drop-in-the-bucket” average. The resulting data grids are designated GRD data arrays. For the "drop-in-the-bucket" gridding algorithm, the key information required is the location of the measurement. The center of each measurement geolocation is mapped to an output-projected grid cell. All measurements within the specified time period that fall within the bounds of a particular grid cell are averaged. This is the reported TB value for this pixel. Ancillary variables contain the number and standard deviation of included samples. The effective spatial resolution of the GRD product is defined by a combination of the pixel size and spatial extent of the 3dB antenna footprint size (Long and Brodzik, 2016). Figure 6 provides a graphical representation of the coarse resolution of GRD measurements.

Figure 6. GRD 25 km Resolution

Reconstruction Algorithm
Reconstruction algorithms use the effective Measurement Response Function (MRF). The MRF is determined by the antenna gain pattern (which is unique for each sensor and sensor channel, and may vary with scan angle), the scan geometry (notably the antenna scan angle), and the integration period. The latter “smears” the antenna gain pattern due to antenna rotation over the measurement integration period. The MRF describes how much the emissions from a particular receive direction contribute to the observed TB value.

For implementation in the CETB, fine map grid resolutions were selected for each channel according to Table 9.

Addition of missing input data for the first day of the month in the N and S projections

Change of the size of the time dimension from 1 to "UNLIMITED"

Addition of files for dates with no data;

13 June 2017

1 Corrected Morning/Evening classifications for all N and S grids2 Adjusted 89 GHz channel gain threshold3 Corrected processing of T grids to include all data from the beginning of the intended calendar day4 Corrected LTOD split time metadata of the N and S grids
Corrected the split times for the N grids which were off by 2 hours5 Corrected processing of data occurring during the calendar year crossover6 Corrected ascending/descending scanline pass classification logic in T grids

11 May 2018

F08

Not available

First release at version 1.1

10 March 2017

Corrected the handling of QC flags to retain more data, eliminating only the flagged data in a given channel and taking care not to eliminate data in other channels.

12 July 2017

2 Adjusted 85 GHz channel gain threshold3 Corrected processing of T grids to include all data from the beginning of the intended calendar day5 Corrected processing of data occurring during the calendar year crossover6 Corrected ascending/descending scanline pass classification logic in T grids

10 June 2018

F10

Not available

First release at version 1.1

14 April 2017

Corrected the handling of QC flags to retain more data, eliminating only the flagged data in a given channel and taking care not to eliminate data in other channels

20 September 2017

2 Adjusted 85 GHz channel gain threshold3 Corrected processing of T grids to include all data from the beginning of the intended calendar day4 Corrected LTOD split time metadata of the N and S grids5 Corrected processing of data occurring during the calendar year crossover6 Corrected ascending/descending scanline pass classification logic in T grids

10 June 2018

F11

Not available

First release at version 1.1

24 April 2017

Corrected the handling of QC flags to retain more data, eliminating only the flagged data in a given channel and taking care not to eliminate data in other channels

03 August 2017

2 Adjusted 85 GHz channel gain threshold3 Corrected processing of T grids to include all data from the beginning of the intended calendar day5 Corrected processing of data occurring during the calendar year crossover6 Corrected ascending/descending scanline pass classification logic in T grids

08 June 2018

F13

Not available

First release at version 1.1

11 April 2017

Corrected the handling of QC flags to retain more data, eliminating only the flagged data in a given channel and taking care not to eliminate data in other channels

11 August 2017

2 Adjusted 85 GHz channel gain threshold3 Corrected processing of T grids to include all data from the beginning of the intended calendar day5 Corrected processing of data occurring during the calendar year crossover6 Corrected ascending/descending scanline pass classification logic in T grids

08 Jun 2018

F14

Not available

First release at version 1.1

03 May 2017

Corrected the handling of QC flags to retain more data, eliminating only the flagged data in a given channel and taking care not to eliminate data in other channels

28 August 2017

2 Adjusted 85 GHz channel gain threshold3 Corrected processing of T grids to include all data from the beginning of the intended calendar day4 Corrected LTOD split time metadata of the N and S grids5 Corrected processing of data occurring during the calendar year crossover6 Corrected ascending/descending scanline pass classification logic in T grids

05 June 2018

F15

Not available

Not available

First release at version 1.2

26 June 2017

2 Adjusted 85 GHz channel gain threshold3 Corrected processing of T grids to include all data from the beginning of the intended calendar day4 Corrected LTOD split time metadata of the N and S grids5 Corrected processing of data occurring during the calendar year crossover6 Corrected ascending/descending scanline pass classification logic in T grids

07 June 2018

F16

Not available

Not available

First release at version 1.2

05 July 2017

2 Adjusted 91 GHz channel gain threshold3 Corrected processing of T grids to include all data from the beginning of the intended calendar day4 Corrected LTOD split time metadata of the N and S grids5 Corrected processing of data occurring during the calendar year crossover6 Corrected ascending/descending scanline pass classification logic in T grids7 Corrected the LTOD in most N and S grids due to orbital drift

24 May 2018

F17

Not available

Not available

First release at version 1.2

06 June 2017

2 Adjusted 91 GHz channel gain threshold3 Corrected processing of T grids to include all data from the beginning of the intended calendar day5 Corrected processing of data occurring during the calendar year crossover6 Corrected ascending/descending scanline pass classification logic in T grids

22 May 2018

F18

Not available

Not available

First release at version 1.2

29 June 2017

2 Adjusted 91 GHz channel gain threshold3 Corrected processing of T grids to include all data from the beginning of the intended calendar day5 Corrected processing of data occurring during the calendar year crossover6 Corrected ascending/descending scanline pass classification logic in T grids

15 May 2018

F19

Not available

Not available

First release at version 1.2

23 June 2017

2 Adjusted 91 GHz channel gain threshold3 Corrected processing of T grids to include all data from the beginning of the intended calendar day5 Corrected processing of data occurring during the calendar year crossover6 Corrected ascending/descending scanline pass classification logic in T grids

11 May 2018

SMMR

Not available

Not available

First release at version 1.2

26 September 2017

1 Corrected Morning/Evening classifications for all N and S grids3 Corrected processing of T grids to include all data from the beginning of the intended calendar day4 Corrected LTOD split time metadata of the N and S grids5 Corrected processing of data occurring during the calendar year crossover6 Corrected ascending/descending scanline pass classification logic in T grids

14 June 2018

1 Grids were incorrectly classified as Evening data and vice-versa. For example, Morning passes for day N were mislabelled as Evening passes for day N-1.

2 All the data from the affected channel were processed with a gain threshold of 8 dB resulting in an excess of missing pixels. The threshold was adjusted to 12 dB to minimize missing pixels.

3 Daily T grid processing did not include final orbit from prior calendar day, which may have included data from current day at the end of the swath. The worst case would have eliminated up to 34 minutes of data at the beginning of the day; however in most cases, it was much less than that.

4 The data in all of the N and S grids were correctly processed into LTOD Morning and Evening divisions using the values in Table 2 in the section on Temporal Coverage, but were reported incorrectly in the metadata. The error occurs only for split times that were different from 0.0-12.0-12.0-24.0. Where the errors occur, they are for the start time in the morning grids and for the end time in the evening grids. In each case the incorrect value is set to 0.0. As an example if, the LTOD split times were 0300 and 1500, then the morning start time was incorrectly reported as 0.0 and the end time was correctly reported as 15.0. Similarly the evening start time was correctly reported as 15.0 and the evening end time was incorrectly reported as 0.0.

The affected metadata fields are:
TB:temporal_division_local_start_time for M (morning) files in the N or S projections
TB:temporal_division_local_end_time for E (evening) files in the N or S projections

5 At calendar year crossovers, data from prior year (Dec 31) were not included in Jan 1 data and data from the following year (Jan 1) were not included in Dec 31.

6 Under rare but not impossible conditions, occasional ascending scanlines were incorrectly classified as descending, and vice-versa.

7 Static LTOD split times were used for the duration of the F16 record, but orbital drift required changing split times. LTOD split times became progressively more offset over the lifetime of the sensor, eventually using values that were incorrect by 5 hours resulting in morning data being incorrectly classified as evening data and vice-versa.